Hello
My latest project is a Beomaster 5500 with the Master Control Panel 5500.
The MCP5500 uses a rotating knob to adjust things like Volume, Balance, etc. The knob has a printed black and white pattern around the perimeter to indicate movement which is read by a reflective optical sensor. The direction of rotation is read by a momentary contact SPDT switch triggered by an arm which slides against the backside of the rotating knob.
On my example, everything possible was wrong with the knob. The plastic knob itself is cracked, the 16005 ball bearing on which the knob rotates was clogged with dirt and would not rotate and the reflective optical sensor, a Siemens SFH900-2 no longer functioned. At least the printed black and white strip didn't fall off, and was the only thing holding the knob together.
First thing was to restore function to the optical sensor. The Siemens SFH900 is no longer available, but is merely an infrared LED and a phototransistor combined into one compact part with the LED's cathode and the phototransistor's emitter tied together to a common pin.
Many modern generic reflective optical senors are available. I chose a generic RPR-220 as the width is about correct and the leads are laid out so that if the cathode and emitter are joined together on one side, the anode and collector are available on the other side. A little creative bending of leads and it fits in place of the SFH900-2. At first it refused to work, but positioning it 1mm further away from the knob coaxed it to life.
Next, removing and cleaning the 16005 bearing made it useable once more. I also obtained a spare bearing, having glued the original knob to the original bearing.
Next, out with some calipers and Fusion 360. A few minutes later, I designed a new knob and 3D printed it. After a few tries I got a good one. Then I arranged a series of black 9mm x 8mm rectangles into a strip and printed it onto a DYMO adhesive label, and attached it to the new knob.
And it all works! Checking the electrolytic capacitors revealed correct capacitance on all but one, which I replaced. The MCP 5500 is now working correctly and controls the Beomaster as expected. Now on to the Beomaster! And the Beocord... and the Beogram CD....
